Large-scale seroprevalence and risk factors associated with Bluetongue virus in Iran

Bluetongue virus (BTV) remains as an economically major concern in the world. Seroprevalence and potential risk factors of BTV were assessed in a cross-sectional study at both the herd and animal levels in Iran. A total of 73 Epidemiologic Units (E.Unit), defined as a herd, flock or village including animals with equal chance of exposure to infectious agents, were randomly selected. Serum samples from all animals (n = 34,575) within the E.Units were collected and tested for BTV sero-group antibodies by using commercially competitive ELISA test. Using cluster analysis, 90.41 % (95 %, CI: 80.85 %–95.47 %) of the E.Units and 56.13 % (95 % CI: 55.61 %–56.66 %) of the tested animals were detected seropositive against BTV. A seroprevalence rate of 57.59 % (95 % CI: 48.01 %–66.63 %), 65.65 % (95 % CI: 59.10 %–73.74 %) and 27.63 % (95 % CI: 14.40 %–46.43 %) was estimated for sheep, goats and cattle, respectively.At E.Unit (herd) level, density was identified as a great risk factor for the infection (r² = 0.891; P = 0.000), and particularly density of cattle significantly correlated with BTV infection within the E.Units (r² = 0.247; P = 0.019). Using multilevel logistic regression, adjusted odds ratios (ORs) were estimated at individual level. A significantly less risk of BTV infection was evaluated for cattle than for sheep (OR = 0.42, 95 % CI: 0.38−0.47, P < 0.001), while no significant difference was observed between sheep and goat (OR = 1.03, 95 % CI: 0.97–1.10, P = 0.345). Animals over 2 years and between 6 months and 2 years expressed 2.22 (OR = 2.22, 95 % CI: 1.96−2.52, P < 0.001) and 2.18 (OR = 2.18, 95 % CI: 1.92−2.49, P < 0.001) times higher chance for the infection than animals under 6 months. Males were at significantly less risk of the infection than females (OR=0.68, 95 % CI: 0.63−0.74, P < 0.001). Animals kept in industrial farming systems displayed 0.46 (OR=0.46, 95 % CI: 032−0.66, P < 0.001) times less chance than animals kept in traditional farming system for BTV, while animals lived in semi-industrial farming system were found to be at 2.97 (OR=2.97, 95 % CI: 2.41−3.66, P < 0.001) times higher chance for BTV than animals lived in traditional farming system. Furthermore, seropositive animals exhibited a high amount of antibodies against BTV (s) suggesting that viral exposure may have frequently occurred during their lifetimes. This large - scale study yielded information on epidemiology of BTV in Iran that is prerequisite for further research, and also for evaluation of any cost-benefit control measure to be established in an enzootic zone of the virus.